Who's at risk
There may also be a genetic factor involved in PTSD. For example, having a parent with a mental health problem is thought to increase your chances of developing the condition.
Why does it develop?
Although it's not clear exactly why people develop PTSD, a number of possible reasons have been suggested.
One suggestion is that the symptoms of PTSD are the result of an instinctive mechanism intended to help you survive further traumatic experiences.
For example, the flashbacks many people with PTSD experience may force you to think about the event in detail so you're better prepared if it happens again.
The feeling of being "on edge" (hyperarousal) may develop to help you react quickly in another crisis.
But while these responses may be intended to help you survive, they're actually very unhelpful in reality because you cannot process and move on from the traumatic experience.
High adrenaline levels
Studies have shown that people with PTSD have abnormal levels of stress hormones.
Normally, when in danger, the body produces stress hormones like adrenaline to trigger a reaction in the body.
This reaction, often known as the "fight or flight" reaction, helps to deaden the senses and dull pain.
People with PTSD have been found to continue to produce high amounts of fight or flight hormones even when there's no danger.
It's thought this may be responsible for the numbed emotions and hyperarousal experienced by some people with PTSD.
Changes in the brain
In people with PTSD, parts of the brain involved in emotional processing appear different in brain scans.
One part of the brain responsible for memory and emotions is known as the hippocampus.
In people with PTSD, the hippocampus appears smaller in size.
It's thought that changes in this part of the brain may be related to fear and anxiety, memory problems and flashbacks.
The malfunctioning hippocampus may prevent flashbacks and nightmares being properly processed, so the anxiety they generate does not reduce over time.
Treatment of PTSD results in proper processing of the memories so, over time, the flashbacks and nightmares gradually disappear.